Genes and proteins updated
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Genes and Proteins
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Genetic Role and Structure of DNA
• Hershey and Chase experiments– Showed that viruses inject their DNA into bacteria
and direct bacteria to replicate it for them• DNA not protein is genetic material
• DNA– Double helix– Made of EQUAL amounts of nucleotides: Adenine,
Guanine, Cytosine, Thymine– Each part of helix is complementary to other, run in
opposite directions– 3 prime and 5 prime ends
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Genetic Role and Structure of DNA• Hershey and Chase experiments and Rosalind Franklin– Showed that viruses inject their DNA into bacteria and
direct bacteria to replicate it for them• DNA not protein is genetic material
Alfred Hershey Alfred Hershey
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The Structure of DNA (review)
DNA has (a) a double helix structure and (b) phosphodiester bonds. The (c) major and minor grooves are binding sites for DNA binding proteins during processes such as transcription (the copying of RNA from DNA) and replication.
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Base Pairing
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Double Helix Structure
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DNA vs. RNA• Same basic structure• Instead of thymine use uracil to pair with adenine• Single helix• Helps code for and make proteins• Types of RNA– Messenger RNA (mRNA) – carries info. For a specific
protein. Segment is codon– Ribosomal RNA (rRNA) – combines with proteins to
form ribosome– Transfer RNA (tRNA) – connectors to bind an mRNA
codon to a specific RNA
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DNA vs. RNADNA encodes the genetic instructions used in development and function; transfers to daughter cells
RNA plays an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals
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The Genetic Code• Genome – All the genetic material in cells– All different sizes depending on complexity of
organisms• Chromosome – Package of DNA and associated
proteins– You have 23 pairs or 46 total chromosomes
• Gene – sequence of DNA on a chromosome that codes for a specific protein or RNA molecule
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Protein Synthesis
• Transcription – Copying a gene’s DNA to a complementary RNA molecule, occurs in nucleus
• Translation – Copying translating an mRNA strand into the language of amino acids
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Transcription and Translation
Transcription occurs within the nucleus; translation occurs outside (still within the cell)
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Transcription• Transcription – Copying a gene’s DNA to a
complementary RNA molecule, occurs in nucleus– Just copying the “words”
• Occurs in 3 Steps– Initiation – Enzymes unzip DNA double helix, RNA
polymerase binds to promoter• Promoter – DNA sequence at the beginning of a gene
– Elongation – RNA polymerase, adds nucleotides from 3’ to 5’ end making RNA molecule
– Termination – RNA polymerase gets to termination sequence at end of gene, separates and releases new RNA molecule
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Transcription
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Preparation for Translation
• Bacteria and archaea begin translation as RNA molecule is being transcribed
• Eukaryotic cells – mRNA can’t cross nuclear membrane– Add nucleotide cap to end of 5’ end of mRNA– Add 100-200 adenines – “poly a-tail”– Helps ribosomes attach to 5’ end of mRNA– Helps prevent degradation of mRNA– Introns removed from RNA, exons spliced together
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The mRNA then leaves the nucleus of the cell; ribosomes translate from mRNA.
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Translation
• Translate DNA/RNA language into a amino acid language to make protein
• Uses– mRNA – carries codon information– tRNA – binds to mRNA and amino acid– Ribosome – anchors mRNA
• Functional Unit = Codon – 3 base pair “word” that coincides with an amino acid– Genetic Code
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Large subunit• 5,080 RNA bases • ~49 proteins
Small subunit• 1,900 RNA bases • ~33 proteins
Small subunit
Large subunit
Ribosome
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tRNA
Tertiary structure of tRNA. CCA tail in yellow, Acceptor stem in purple, Variable loop in orange, D arm in red, Anticodon arm in blue with Anticodon in black, T arm in green.
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tRNA and the ribosome combined
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Triplet Codon
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Translation
• Occurs in 3 Steps– Initiation – at 5’ end “start” codon (AUG) codes for
methionine, calls large subunit, start polypeptide– Elongation – tRNA brings 2nd amino acid,
covalently bonds with 1st amino acid, release 1st tRNA, get another tRNA and so on to make poly peptide chain
– Termination – “Stop” codon (UGA, UAG, or UAA). NO amino acid corresponds to “stop”, release factors, release last tRNA, ribosomal units separate, polypeptide chain released
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Steps in Translation
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Steps in Translation
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Polypeptide to Protein
• Polypeptide chain is NOT a protein• Chain folds in cytoplasm to get 3-D structure• Errors can occur– Wrong amino acid sequence “messes” up folding• Cystic fibrosis
– Error in folding with correct sequence• Alzheimer Disease – incorrect folding of amyloid, forms
mass in brain
– Error in joining polypeptide chains• Misfire in types or how joined. Hemoglobin
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How Do Prokaryotic Cells Express Proteins?
• Operons
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How Do Prokaryotic Cells Express Proteins?
• Operons – Used by bacteria– Group of genes plus promoter and
operator/repressor• Operator – DNA sequence between promoter and
protein encoding region• Repressor – Protein that binds to operator to inhibit
transcription
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How Do Prokaryotic Cells Express Proteins?
• Lac Operons: lactose turns off the repressor
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How Do Prokaryotic Cells Express Proteins?
• TRP Operon : presence of TRP = active repressor
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How Do Eukaryotic Cells Express Proteins?
• All cells in an organism contain identical DNA sequences.– i.e. cloning experiments
on plants
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How Do Eukaryotic Cells Express Proteins?
• All cells in an organism contain identical DNA sequences.– i.e. cloning experiments on animals
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How Do Eukaryotic Cells Express Proteins?
• Transcription Factors– Eukaryotic Cells– Bind to DNA at different sequences to control
transcription– Can bind to promoter or enhancer– Respond to external stimuli to signal gene to “turn on”
• Signaling molecule binds to outside of cell, triggering reactions inside
– Defects can cause disease or be used as drugs• Cancer• RU486
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How Do Eukaryotic Cells Express Proteins?
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How Do Eukaryotic Cells Express Proteins?• DNA Availability– If DNA not “unwound” from double helix, cannot do
transcription• Molecules bind to DNA and either not allow to unwind or wind it
even tighter
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How Do Cells Express Proteins?
• RNA Processing– Removal of introns to change what proteins are
coded for
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How Do Eukaryotic Cells Express Proteins?
• Methylation of certain portions of DNA can deactivate those genes
• X-inactivation
The coloration of tortoiseshell and calico cats is a visible manifestation of X-inactivation. The black and orange alleles of a fur coloration gene reside on the X chromosome. For any given patch of fur, the inactivation of an X chromosome that carries one gene results in the fur color of the other, active gene.
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Mutations
• Mutation – Change in cell’s DNA sequence• Not always harmful, can lead to genetic variability• Point Mutation – changes 1 or a few base pairs in a
gene– Substitution – replacement of 1 DNA base pair with
another• Silent – mutation codes for same protein• Missense – mutation codes for different amino acid, changing
proteins shape (ex. Sickle cell anemia)• Nonsense – mutation codes for “stop” codon instead of amino
acid – makes shorter peptide chain
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Mutations: Substitution
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Missense
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Mutations
• Base Insertions and Deletions– 1 or more nucleotides are added or subtracted from gene
• Frameshift Mutation – adds or deleted nucleotides in any number other than multiple of 3– Disrupts codon reading, alters amino acid sequence
• Expanding Repeats– Number of a 2 or 4 nucleotide sequence increases over
several generations• Symptoms get more and more severe• Huntington’s Disease – makes extra glutamines, makes fibrous
clumps in brain
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Mutations: Deletions
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Mutations: Deletions
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How are these Mutations Produced?
• Unmatched base pairs can possibly lead to changes in the DNA structure
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How are these Mutations Produced?
• Unmatched base pairs can possibly lead to changes in the DNA structure
Wild-Type Sequence
DNA CTG ACT CCT GAG GAG AAG TCT
Protein Leu Thr Pro Glu Glu Lys Ser
Amino Acid Position
3 4 5 6 7 8 9
Sickle Cell Sequence
DNA CTG ACT CCT GTG GAG AAG TCT
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Mutations
• Causes– Spontaneous – DNA replication error– Mutagens – external agent that induces mutations
• UV Radiation, x-rays, chemical weapons, nuclear energy, tobacco
– During Meiosis– Transposons – jumping pieces of DNA
• Types– Germline – occurs in cells that give rise to sperms and eggs
• Things that run in families
– Somatic – occurs in non-sex cells• DOES NOT get passed on